The invention relates to a process for the production of photopolymerizable cylindrical, continuous seamless flexographic printing elements by applying a layer of a photopolymerizable material to the outer surface of a hollow cylinder and joining the edges by calendering. The invention furthermore relates to an apparatus suitable for carrying out the process.
Cylindrical flexographic printing plates are known in principle. In a cylindrical flexographic printing plate, the printing cylinder of the printing press is provided over the whole circumference with a printing layer or a print relief. Cylindrical printing plates are of considerable importance for the printing of continuous patterns and are used, for example, for the printing of wallpapers, decorative papers or gift-wrapping papers.
In principle, the actual printing cylinder of the printing press can itself be provided with a printing layer which completely surrounds it. However, this procedure has the disadvantage that the entire printing cylinder has to be replaced in certain circumstances on changing the printing plate. This is extremely complicated and accordingly expensive.
The use of so-called sleeves is therefore customary. Sleeves comprise a cylindrical hollow body which has been provided with a printing layer or a print relief. The sleeve technique permits very rapid and easy changing of the printing plate. The internal diameter of the sleeves corresponds to the external diameter of the printing cylinder, so that the sleeves can easily be pushed over the printing cylinder of the printing press. Pushing on and moving the sleeves works according to air cushion principle: for the sleeve technology, the printing press is equipped with a special printing cylinder, a so-called air cylinder. The air cylinder has a compressed air connection at the end face, by means of which compressed air can be passed into the interior of the cylinder. From there, it can emerge again via holes arranged on the outside of the cylinder. For mounting a sleeve, compressed air is passed into the air cylinder and emerges again at the outlet holes. The sleeve can now be pushed onto the air cylinder because it expands slightly under the influence of the air cushion, and the air cushion substantially reduces the friction. When the compressed air supply is stopped, the expansion declines and the sleeve fits firmly on the surface of the air cylinder. Further details of the sleeve technique are disclosed, for example, in “Technik des Flexodrucks”, page 73 et seq., Coating Verlag, St. Gallen, 1999.
However, high-quality round printing plates cannot be produced by simply surrounding the printing cylinder or a sleeve with a flexographic printing plate processed ready for printing. In fact, a fine gap which always also intersects printing parts of the plate in the case of a true continuous motif remains at the abutting ends of the printing plate. This gap leads to a clearly visible line in the printed image. In order to avoid this line, only nonprinting depressions may be present at this point. Thus, it is not possible to print any desired patterns. Moreover, there is in this technique the danger that the solvent present in the printing ink may penetrate into the gap and may detach the ends of the printing plate from the printing cylinder. This leads to even greater defects in the printed image. Even when the ends are adhesively bonded, clearly visible traces still remain in the printed image.
For the production of high-quality round printing plates, it is therefore necessary to provide the printing cylinder or a sleeve with a completely surrounding, relief-forming, photopolymerizable layer by means of suitable techniques. Only in a second step is the cylindrical photopolymerizable flexographic printing element as such processed to give the final round printing plate. Apparatuses for processing cylindrical flexographic printing elements are commercially available. The application of a continuous seamless, photopolymerizable layer can be effected, for example, by coating from solution or by ring extrusion. However, both techniques are extremely complicated and therefore correspondingly expensive.
In general, maximum precision is important when applying the photopolymerizable layer: modern photopolymerizable flexographic printing elements permit production of flexographic printing plates having substantially higher resolution than was the case in the past. Flexographic printing is therefore also increasingly making inroads into those areas which were previously the preserve of other printing processes. At higher resolution, however, defects in the printing surface of the flexographic printing plate are also more rapidly visible. Differences in thickness in the relief-forming layer have a considerable adverse effect on the true running of the printing cylinder and hence on the print quality. In the case of high-quality flexographic printing plates, the thickness tolerance should usually be not more than ±10 μm.
If the thickness tolerance of the photopolymerizable layer of the sleeve is not sufficient, the surface of the sleeve has to be refinished. DE-A 31 25 564 and EP-A 469 375 disclose processes for improving the print quality, in which the surface of the cylindrical flexographic printing element is first ground and then smoothed with a suitable solvent, and remaining irregularities are, if appropriate, filled with binder or with the material of the photosensitive layer. Such a procedure is of course extremely complicated and tedious. Accordingly, it is therefore absolutely essential to avoid it in an economical process.
There are also known techniques in which a prefabricated, thermoplastically processable layer of photopolymerizable material is wound around the printing cylinder or the sleeve, and the abutting edges of the photopolymerizable layer, also referred to as a seam, are closed as well as possible by means of suitable techniques. It has been proposed, for example by DE 27 22 896, to bond a commercial, sheet-like, photopolymerizable flexographic printing element together with the substrate film adhesively to a printing cylinder or a sleeve so that the cut edges abut one another. The adhesive bonding is preferably effected by means of a double-sided adhesive film. The cut edges are straight and are subsequently welded to one another under pressure and at elevated temperatures. The welding can be effected with the aid of a heated calender roll. The use of a plate having a substrate film is, however, extremely problematic. Typical substrate films have a thickness of from 0.1 to 0.25 mm. If the substrate film does not completely cover the circumference and, owing to a small error in mounting or in cutting to size, leaves even only a minimum gap, the empty space present between the film ends fills with polymeric material during calendering, and an impression of this gap remains on the surface of the photopolymerizable layer and leads to visible defects in the print. It is therefore also necessary as a rule to regrind and smooth such a flexographic printing element.
DE-A 29 11 908 discloses a process in which a photosensitive resin film is wound around a printing cylinder without a substantial distance or a substantial overlap being present between the plate ends. The application to the cylinder is preferably effected with the use of a doubled-sided adhesive film. The seam is closed by bringing the printing cylinder into contact with a rotating calender roll and joining the cut edges to one another by melting. For heating, DE-A 29 11 908 proposes either heating the calender roll from the inside or heating the photosensitive resin film by means of an IR lamp from the outside. Regarding the temperature, the publication states that the photosensitive material should soften but preferably should not flow.
The commonly assigned, as yet unpublished DE 103 18 042.7 application discloses a process for the production of cylindrical flexographic printing elements wherein a photopolymerizable layer is applied to a sleeve and the seam is joined by means of a calender roll while heating. For this purpose, the total calender roll is preferably heated from the inside. The heating may be supplemented by an IR lamp. The edges to be joined are cut as required by means of miter cuts.
In all documents cited, the photopolymerizable layer is heated in its entirety. As a result of this, however, the sleeve or the printing cylinder, too, and hence also the double-sided adhesive tape which is used for adhesive bonding of the photopolymerizable layer heats up with increasing calendering time. However, the adhesive force of the adhesive tapes decreases with increasing temperature so that the photopolymerizable layer is fixed on the sleeve only with a relatively small force and may slip. This results in a less smooth surface, so that the surface once again has to be refinished. Furthermore, a layer may also undergo plastic deformation in an undesired manner in the case of an excessively high temperature.
In addition to the problem of a high-quality seam closure and the obtaining of a layer thickness which is as constant as possible, preexposure from the back prevents a further problem of sleeve technology. Flexographic printing elements are usually preexposed before the actual main exposure from the back through the substrate film for a short time span. As a result of this, the relief background is prepolymerized and better anchoring, in particular of fine relief elements, in the relief background is achieved.
In the case of sleeves, preexposure from the back is as a rule not possible since the conventional sleeve materials, such as, for example, glass fiber-reinforced plastic or metal, are not transparent to UV radiation. EP-A 766 142 has proposed the use of transparent sleeves, in particular sleeves of polyesters, such as PET or PEN, in a thickness of from 0.25 mm to 5 cm. However, these are expensive. Furthermore, special exposure units for uniform exposure of the sleeve from the inside are required. Moreover, the person skilled in the art is faced with a typical dilemma in the case of transparent sleeves. The mechanical stability of the sleeve increases with increasing thickness of the sleeve whereas the transparency of the sleeve to actinic light decreases with increasing thickness of the sleeve. The problem of efficient exposure of sleeves from the back without reducing the stability of the sleeve is still unsolved.
It is possible in principle to preexpose a solid photopolymerizable layer from the back even before application of the sleeve. However, layers preexposed in this manner could not to date be welded as satisfactorily as would be expedient and necessary for the production of high-quality continuous seamless printing plates, because, as is known, only the uncrosslinked polymer layer, but not the exposed, crosslinked polymer layer, can be satisfactorily welded. Furthermore, the effect of the preexposure is frequently lost again as a result of the welding of the layer ends at elevated temperatures. Consequently, in particular fine relief dots are poorly anchored.
For solving this problem, DE-A 37 04 694 has therefore proposed firstly applying a first layer of photopolymer material to a sleeve, welding the seam and then polymerizing the photopolymeric layer from the front by exposure to light. In a second process step, a photopolymeric layer is applied to the first, already crosslinked layer and the seam thereof is also welded. This two-stage process is, however, inconvenient and expensive.